Hurricane Isabel (2003): New Insights into the Physics of Intense Storms. Part I: Mean Vortex Structure and Maximum Intensity Estimates

Abstract

This study is an observational analysis of the inner-core structure, sea surface temperature, outflow layer, and atmospheric boundary layer of an intense tropical cyclone whose intensity and structure is consistent with recent numerical and theoretical predictions of superintense storms. The findings suggest new scientific challenges for the current understanding of hurricanes. Unprecedented observations of the category-5 Hurricane Isabel (2003) were collected during 12–14 September. This two-part article reports novel dynamic and thermodynamic aspects of the inner-core structure of Isabel on 13 September that were made possible by analysis of these data. Here, a composite of the axisymmetric structure of the inner core and environment of Isabel is estimated using global positioning system dropwindsondes and in situ aircraft data. In Part II, an extreme wind speed observation on the same day is discussed in the context of this work. The axisymmetric data composite suggests a reservoir of high-entropy air inside the low-level eye and significant penetration of inflowing near-surface air from outside. The analysis suggests that the low-level air penetrating the eye is enhanced thermodynamically by acquiring additional entropy through interaction with the ocean and replaces air mixed out of the eye. The results support the hypothesis that this high-entropy eye air “turboboosts” the hurricane engine upon its injection into the eyewall clouds. Recent estimates of the ratio of sea-to-air enthalpy and momentum exchange at high wind speeds are used to suggest that Isabel utilized this extra power to exceed the previously assumed intensity upper bound by 10–35 m s−1 for the given environmental conditions. Additional study with other datasets is encouraged to further test the superintensity hypothesis.

Tropical‐cyclone intensification and predictability in three dimensions

“…The tendency of the simulated intensity to increase substantially at higher resolution has been documented in the literature (Persing and Montgomery, 2003;Hausman et al, 2006), but a detailed examination of this issue in three dimensions is beyond the scope of this work (This simulated hurricane and that of the control experiment have been confirmed to be 'superintense' as defined by Persing and Montgomery (2003). These numerical experiments support the hypothesis that the superintensity phenomenon persists in three dimensions (Montgomery et al, 2006b).). At the beginning of rapid intensification in experiment S5 (Figure 18(a)), many more VHTs form in an annular region than in the control experiment.…”

Specifying Air-Sea Exchange Coefficients in the High-Wind Regime of a Mature Tropical Cyclone by an Adjoint Data Assimilation Method

“…Even qualitatively, there is no consensus on the behavior of C K for winds over 30 m s . In terms of the estimates of maximum potential intensity based on the WISHE mechanism, the ratio C K /C D is thought to be more uncertain than other parameters such as sea surface temperature (SST), outflow temperature, and ambient relative humidity (Montgomery et al 2006). …”

Where do winds come from? A new theory on how water vapor condensation influences atmospheric pressure and dynamics

“…mixing (Montgomery et al, 2006). Above approximately 1.5 km the atmosphere within and outside the hurricane is approximately isothermal in the horizontal direction (Montgomery et al, 2006, Fig.…”